Plethysmography is an optical method of obtaining a so-called plethysmogram which provides information about the pulse frequency and the oxygen saturation of the blood of a test person. A plethysmogram is a graphic illustration of volume changes. In this field of application, it is specifically the volume changes of an arterial blood flow at a localized measurement site of the human body which are detected as the plethysmogram. To implement this in technical terms, light is radiated through tissue at a body location having arterial blood vessels. The patient has a sensor applied to him/her which contains a light source and a photoreceiver, so that the light passes through the tissue layer, and so that the remaining light intensity impinges upon the photoreceiver. Within the body, the light undergoes an attenuation which is dependent, among other things, on the wavelength of the light source, the type and concentration of the substances in the irradiated tissue, and on the pulsation of the blood. The signal of the photoreceiver which has thus been obtained is present in the form of a photocurrent, is dependent on the above-mentioned general conditions, and corresponds, in a first approximation, to the changes in the blood volume of arterial vessels which are caused by contraction of the heart muscle. FIG. 25 shows the basic architecture of an apparatus for detecting a plethysmogram. A microcontroller (μC) controls, via two driver stages, two LEDs of different wavelengths, one light source being sufficient, in principle, for creating a plethysmogram. The LEDs depicted in FIG. 25 emit light in the red and infrared regions. The light emitted by the LEDs subsequently passes through the tissue of the test person, in FIG. 25 this is depicted, by way of example, as a finger. Once the light has passed through the tissue of the test person, it will impinge upon a photosensor. The photsensor converts the optical signals into electrical signals and passes them on to processing electronics which amplify the signal, convert them from analog to digital and feed them to the microcontroller (μC). The microcontroller (μC) then determines two plethysmograms, one plethysmogram for each wavelength, from the digital signals fed to it. From the waveforms of the plethysmograms thus measured, physiological parameters, such as the heart frequency or the oxygen saturation of the test person's blood, may be measured, with one single plethysmogram being sufficient, in principle, for determining the heart frequency, for determining the oxygen saturation of the blood, two plethysmograms of light sources of different wavelengths being necessary.
Pulse oximetry is a non-invasive method of measuring the oxygen saturation of the blood (SpO2) and the heart frequency (HR) by means of an optical sensor. The oxygen saturation detected by the pulse oximeter is specifically referred to as the SpO2 value. The oxygen saturation is defined as the ratio of the concentration of oxygen-saturated hemoglobin molecules and the overall hemoglobin concentration, and is indicated in percent. A component of the pulse oximeter is a sensor having two integrated light sources and being configured similar to a plethysmograph, cf. FIG. 25. In pulse oximetry, use is made of at least two plethysmograms to determine the color of the arterial blood. The color of the blood, in turn, is dependent on the oxygen saturation. By selecting the wavelengths of the light sources well, it may be shown that a quantity correlating well with the oxygen saturation may be obtained from the ratios of prominent points within the plethysmogram. Typically, the spectra of the receive signals of two light sources of different wavelengths are determined, and the quotient of specific spectral values is formed. This quotient will then be approximately proportional to the SpO2 value of the blood.
An essential quality characteristic in comparing pulse oximeters is the resistance toward interferences. Filtering those unuseful signal portions which arise because of the movement of the patient is particularly problematic. Even with small movements, the amplitudes of the motion artifacts may seem larger than those of the pulse wave within the signal. If the signal is highly overlaid by motion artifacts, this will lead to a temporary operational failure of the equipment, with this problem being signaled accordingly. In the worst case, the equipment will not detect the distorted measurement and will not issue a signal, so that the measurement values indicated will erroneously be held as true. The quality of treatment of a patient may be clearly reduced due to measurement values being incorrectly indicated. Especially in the environment of operating rooms, the above-mentioned distortions represent a major disadvantage of pulse oximeters.
In addition to the motion artifacts, high-power light sources, such as those of operating-room lamps, fluorescent lamps or monitors, may cause unwanted interferences in the signal. With conventional pulse oximeters or plethysmographs, this problem is typically diminished by introducing additional measurement periods for determining the ambient light, and by subsequently subtracting the ambient-light measurement from the useful-signal measurement. During these measurement periods or time slots, all light sources of the sensor are switched off, and only the ambient light is measured. The ambient-light intensity is subtracted from the plethysmogram, and thus the portion of ambient light is largely separated from the pulse signal. However, especially with pulsating or AC-powered ambient-light sources, an interference portion will remain within the plethysmogram. The interference portion within the plethysmogram thus highly depends on the electronic equipment, or interferers, used in the surroundings. Especially in the intensive care of patients, a multitude of electronic devices and tools are employed, so that the susceptibility of pulse oximeters and plethysmographs to interference is a given fact particularly in intensive-care environments. Particularly in the field of intensive care, however, measurement errors of physiological parameters such as the heart frequency or the blood oxygen saturation are extremely critical and may entail serious consequences.
In pulse oximetry, transmission and remission sensors have several LEDs (transmitters) and only one photodiode (receiver). The test person's tissue is irradiated by LEDs of different wavelengths, and the photodiode receives the light of different wavelengths from the tissue. In principle, it would be possible to differentiate various channels by means of the wavelengths of the LEDs, e.g. by color filters present at several photodiodes. Since this involves a large amount of technical expenditure on the side of the photodiode, the intensities of the LEDs must be modulated. Only then is it possible to differentiate between the wavelengths by means of a single broad-band photodiode.
In order to enable the receiver to differentiate between various transmit sources (LEDs) having different wavelengths, TDMA concepts (time division multiple access) are employed with known pulse oximeters. Each sensor LED has a time window assigned to it within which it is switched on. FIG. 26 illustrates this time sequence of signals. One may recognize that the various LEDs successively have time slots of equal durations associated with them which are separated by dark periods of equal durations. FIG. 26 shows a schematic sequence with three different LEDs. The LEDs of different wavelengths successively light up for a short time duration, in FIG. 26, the bright periods of the LEDs are designated by “LED 1”, “LED 2”, and “LED 3”. Typical frequencies with which the light sources of current pulse oximeters are controlled amount to 20-50 Hz. By adding additional dark phases during which none of the LEDs lights up, designated by “DARK” in FIG. 26, one tries to measure the signal portion caused by ambient light, and to subsequently subtract it from the useful signal. Nevertheless, the results are often distorted by ambient light or by high-frequency surgery influences. In high-frequency surgery, tissue is cut by means of high-frequency voltages. These high frequencies cause inductions in lines of the pulse oximeters and may thus interfere with their functioning. The local influences may be largely suppressed, since the sensors are protected against irradiation from the outside. Nevertheless, ambient light will enter into the shell of the sensor.
A further example of dynamic interferences may be found with test persons who have long-term measurements conducted on them. They wear a sensor with integrated LEDs and a photoreceiver over a relatively long time period for detecting long-term data. These patients or test persons, for example during car journeys through tree-lined streets or streets lined by many high buildings, are subject to pronounced and, as the case may be, rapid changes in the lighting conditions. In places, these changing lighting conditions express themselves in a manner very similar to the interferences in in-patient environments of hospitals. In principle, test persons subjected to long-term measurements are exposed to a multiplicity of ambient-light influences which may give rise to a whole spectrum of interferences.
The signal quality is clearly improved by subtracting the ambient-light portion, determined by adding dark phases. However, interference artifacts will remain which may lead to incorrect SpO2 values. Up to now, it has not been possible, despite numerous attempts, to remove those interferences, which are caused by fluorescent lamps, infrared heat lamps, operating-room illumination and monitors, from the useful signal. Since in pulse oximeters and plethysmographs, the ratio between useful signals, i.e. those signal portions caused by the change in volume of the tissue, and the interferences may be very unfavorable, those interferences which are distorted further by signal processing are also relevant. For example, prior to an analog/digital conversion, signals are low-pass filtered to avoid errors caused by subsampling. Since the filters used only ever have a finite attenuation within the stop band, errors caused by subsampling, also referred to aliasing errors, will nevertheless arise. Depending on the original interfering frequency, these interferences will then be mirrored into the useful range, where they may occur at different frequencies.
The susceptibility of current pulse oximeters and plethysmographs to interferences will rise if the above-mentioned interferers are located within their surroundings. Especially in operating rooms or intensive-care units, there are a multiplicity of electronic devices, or electronic interferers. Particularly in such environments, thus, the susceptibility of current pulse oximeters and plethysmographs to interferences increases. This significant disadvantage may entail serious consequences for test persons if such situations give rise to measurement errors which cannot be immediately identified as such.
Known plethysmography methods may be found in the following documents, for example:
EP 1374764 A1/WO 2002054950 A08, which describes a basic circuit for measuring and detecting a plethysmogram, and deals with the above-described signal processing in detail.
EP 208201 A2/A3, wherein optical detection of a change of volume of a body part, and an evaluation device for evaluating the optical signals are protected, in principle. The method described there makes use of the changing outward volume change of extremities caused by the pulse and the changes in blood pressure associated therewith.
EP 341059 A3. Here, a basic pulse oximetry method is described which exploits light sources (LEDs) of different wavelengths. Light of different wavelengths is radiated through the test person's tissue, the light signals are absorbed from the tissue by means of optical sensors and are evaluated by a corresponding analog signal processing.
EP 314331 B1, a pulse oximetry method also based on light of different wavelengths is used for radiating through the tissue of a test person. The optical signals thus obtained are converted to electric signals, and a value which provides insights into the oxygen saturation of the test person's blood is extracted therefrom.
EP 1254628 A1, the pulse oximeter protected here is also configured to determine oxygen saturation of blood, the method proposed here additionally reducing interferences caused by cross-talk.
U.S. Pat. No. 5,503,144/U.S. Pat. No. 6,714,803, here a description is given of signal processing methods for linear regression which determine an SpO2 value by means of two plethysmograms. A correlation coefficient which serves as the reliability measure is determined from among the two plethysmograms.